Without knowing how your VCOs are designed, my experience has been microphonics are
usually caused by mechanical resonances in the structure of the VCO resonator and
adjacent metal objects causing capacitance changes. The changes in resonator
capacitance to ground change the VCO center frequency. This is usually caused by a
board with traces or vias adjacent to a shield or chassis parts. It doesn't take
much of a capacitance change to produce modulation on the VCO center frequency.

Measuring the modulation frequency sidebands on the VCO output with a spectrum
analyzer is a good way to troubleshoot the problem. The sidebands produced by
tapping on the structure (an impulsive stimulus) can help estimating the mechanical
resonant frequencies of the structure. Placing an operating unit on a shake table
while monitoring the output frequency with a spectrum analyzer is a good method to
determine what's resonating and at what frequency. A strobe light operating
slightly off the shake table frequency will allow you to observe the mechanical
resonances in slow motion.

The problem can usually be fixed by constraining the board and the metal near the
VCO with stand-offs or something similar. Or installing a small rigid shield over
the VCO resonator, possibly on both sides of the board can eliminate the cap changes
to larger structures.

Another possible source is your reference oscillator. Crystal oscillators can be
microphonic for the above reasons. Movement of the crystal with respect to the case
is frequently an issue. Reference frequency sources can frequently be a big problem
because there is usually some significant frequency multiplication within the PLL,
causing enhancement of noise sidebands on the reference signal.

---Bob Lewandowski
Vixel Corp.

"Liang, Hongbo" wrote:

> Hi gurus:
>
> We met microphonics problem with our frequency source design. I think there
> might be experts
> out there.
>
> Here's our situation: we have four different sources on one FR4 board. Named TX
> PLL, RX PLL,
> TX SYNTHESIZER, RX SYNTHESIZER. The TX and RX used different references.
>
> We could isolate the microphonics problem with TX PLL, which is 2.6GHz. The RX
> PLL uses almost the
> same design as TX PLL and has no problem. Only different is the RX is 1.7GHz.
> Both of them use VCO.
>
> My question as: what's the physics behind microphonics? What're the roots of it?
>
> Wish someone out there could enlightened me.
>
> Best Regards,
> Hongbo Liang
>
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